The use of catalyst in distillation columns to perform simultaneous distillation and catalytic reactions in a single contacting section or the dual functions of distillation and catalytic reaction in different sections of a common column are well known. To improve contacting and lower pressure drops it has been preferred to incorporate the catalytic material into a structured packing material. The structured packing material serves to increase vapor liquid contacting while lowering the pressure drop of vapor through the packed material. Structured packing material may also be useful to prevent flooding of the packed column section containing the catalyst so that true catalytic distillation may occur if desired.
Structured packing in the form of perforated plates sections has been found particularly desirable for use within catalytic distillation zones. The permeable sheets of material are typically formed from thin plates or screens. A shape in the form of corrugations is preferred. The corrugations aid in the dispersion of vapor and also add surface for contacting between vapor and liquid. Plate or screen sections are typically used to facilitate the forming process. Forming techniques have generally limited either the size or thickness of the corrugated packing elements made from plates. Large plates must use easily deformable material such as thin foil-like sheets. The thin plate materials have insufficient strength to support catalyst and maintain flow channels through the packing material. More structurally stable material such as thicker plate elements can only be formed in relatively small sections having width and height dimensions of less than 1 meter. While thinner plate sections can be made to maintain required channel dimensions, the thinner plates must be modified to add additional structural strength by techniques such as joining the corrugations at their contact points. However, such techniques are expensive and greatly complicate the manufacture of the packing elements.
The need to use plate elements having relatively small lateral dimensions as a packing material interferes with the long term usefulness of catalytic distillation arrangements. Effective catalytic distillation zones require at least several meters of height to have adequate reaction time and sufficient stages of separation. It's relatively common to have structured packing sections that contain catalyst with heights of about 9-11 meters and, depending on the diameter of the column, containing more that 1000 ft.sup.3 of catalytic structured packing volume.
With the constrained dimension of the corrugated plate or screened elements, multiple layers of such elements are routinely stacked into the catalytic distillation section of a distillation column. Loading of multiple elements is time consuming. The structured packing units containing catalyst ordinarily have a roughly cube shaped geometry with sides of 12 to 18 inches in length and therefore can occupy as little as 1 cubic foot of distillation volume. Accordingly, for an average size reactive distillation arrangement, there could be more than 1000 of such cubes with many smaller cubes shaped to accommodate the curvature of the usual round column walls.
The stacking of multiple elements also occludes the openings of the channels provided between the corrugations which prevents catalyst from being loaded into the packing once it is in place in the column. Therefore, catalyst is preloaded into the packing modules before their installation into the distillation column. These modules typically retain the catalyst in a sealed envelope between the corrugations. In such arrangements it is not possible to replace catalyst without removing the packing units from the distillation column. Removal and installation of the packing units from the distillation column makes catalyst change-out and catalyst replacement a time consuming operation and an expensive part of continued catalytic distillation operation within the column.
The use of sealed catalyst envelopes has other disadvantages. For example, the number of channels that contain catalyst and are free of catalyst cannot be changed other than by a complete removal, replacement, and reloading of packing material from the column.